This work demonstrates a more accurate method for calculating structural parameter (S) of asymmetric osmotic membranes using experimental data and a theoretical flux model which encapsulates all significant boundary layer phenomena. External boundary layer effects on the porous side of the membrane have been neglected in many current models. In these models, external concentration polarization (ECP) effects get combined with internal concentration polarization (ICP), resulting in inflated S values. In this study, we proposed a mathematical flux model in which ECP effects are accounted for, so that S can be more accurately measured. This model considered the in-series resistances for solute transport based on intrinsic properties of the membrane, as well as boundary layers at membrane surfaces and within the support layer. We therefore introduced new equations to define total resistance to solute transport and reflection coefficient of membranes in FO. The results indicate that ICP is less severe than previously predicted and that cross-flow velocity, temperature and concentration of the draw and the feed solutions impact both external and internal concentration polarization. Our calculations surprisingly show that changes in cross-flow velocity impact internal concentration polarization due to induced mixing within the support layer. Also, we suggest that it is critical to consider the "residence time" of solutes in the vicinity of the selective layer when determining the membrane selectivity. (C) 2015 Elsevier B.V. All rights reserved.